Geochronology is the study of dating of rocks and has been extensively used for dating earth crust and mantle specimens, for example. One way in which this has been done, and the way done according to the preferred embodiment, is to determine a ratio between the 187 isotopes of rhenium and osmium (herein Rh and Os). Rh-187 decays to Os-187 with a half life of 50 billion years. The ratio between the amount of Rh-187 and Os-187 has been used for dating such rocks. Other materials which decay in this way include Ur.fwdarw.Pb, Sm.fwdarw.Nd and Rb.fwdarw.Sr. A brief description of the way this is done will be provided herein.
One of the most accurate ways of determining the amount of Rh and Os present is by using a mass spectrometer. There are many kinds of mass spectrometers, but the two most common in use include a first type which determines a mass of the ion, and a second type which determines a ratio between amount of one isotope to another. The first type mass spectrometer can determine a specific mass to a high accuracy. The second type of mass spectrometer, and the type which is used in the environment of the present invention, provides a very accurate ratio of masses present between two or more isotopes.
The Rh/Os ratio is obtained is by taking a sample to be analyzed and pulverizing it in a way known in the art. This special pulverization causes the sample to break along crystal boundaries so that contiguous crystal amounts are obtained. The ratios of Rh to Os within similar kinds of contiguous crystals will accurately date the age of the rock. The crystal groups are then processed in a way known in the art to separate the Rh and the Os from the rest of the rock. This is beyond the scope of this disclosure. At that point, the amount of Rh and the amount of Os will be determined.
The Rh-187 amount is determined by taking the sample of Rh and diluting it with a sample of pure Rh-185. Such samples are available from, for instance, Oak Ridge Laboratory. The combined sample of Rh-187/Rh-185 is run through the ratio-type mass spectrometer which provides the amounts of 187 relative to 185. Since the amount of Rh-185 which has been added is known, the amount of Rh-187 can then be determined. Similarly, the Os-187 is determined. Rh-187 is radioactive. The sample of Os will, itself, have a number of peaks and the ratio between the peaks which are normally there, and the 187 peak, is used to determine the amount of Os-187. Once the amount of Rh-187 and Os-187 are determined, they are plotted as a function of one another, and the slope of this line provides the age of the sample from which it was taken.
In order to accurately assess the amount of Rh or Os, a ratio between at least two sample beams, each representing an amount of one isotope, must be taken. The problem in the prior art is that for heavy elements, such as Rh and Os, the separation between the beams becomes smaller. The beam separation at the exit of a typical spectrometer used in this way is about 1.4 mm.
The amplitude or amount of these ion beams can be detected in two ways. The first technique of detection of such beams is by using a so-called Faraday cup. A typical Faraday cup is a metal cup with razor blade-like structures defining an entrance into the cup. A wire is attached to each cup, and measures the current caused by the ions or electrons which enter it.
A limitation on the use of a Faraday cup is that they can only register relatively large amounts of current. Faraday cups cannot operate effectively with a current of less than, for example, 10.sup.-12 amperes.
Any current less than 10.sup.-12 amperes requires operation using a so-called electron multiplier. An electron multiplier is conceptually a series of electrodes, each of which produces a plurality of electrons for each electron or ion which impinges thereon. The subsequent electrodes are at lower potential than the earlier electrodes and therefore the electrons impinging on the device are continually increased until the output of the electron multiplier. Electron multipliers of so-called semiconductor glass have also been made.
Faraday cups can be made very small, and in fact, small enough to obtain information from beams split on the order of magnitude of 1.4 mm, such as from Rh and 187 Os. However, electron multipliers are typically required to obtain readings from a small sample of heavy material (e.g. 187). Even the smallest electron multipliers, however, are typically at least an inch in size and have their entrance slit in their center so that they cannot be used to resolve closely adjacent beams.
Accordingly, when multiple beams with small separations that require an electron multiplier have been used in the prior art, the electron multiplier has been moved from beam to beam. Typically, one beam is measured for ten seconds and the other beam is subsequently measured for the next ten seconds. This provides a good approximation of the ratio between the beams, but the problem is that the beam is never totally stable with time and therefore the ratio is inaccurate by whatever instability exists. However, there has been no way known to obviate this problem. This procedure is also wasteful of sample, which is frequently very small, as ions can be registered only when a beam is directed into a detector.
The present invention obviates this problem in a way that is nowhere taught or suggested by the prior art, and discussed in detail herein.